This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Chemical signals detected by the self-referencing technique are inevitably distance dependent, resulting from diffusion of the analyte through the extracellular matrix. Obviously this necessitates the reproducible placement of electrodes at specific distances from the cell. Unfortunately, manual placement of the probe can be quite difficult and is limited to the high contrast boundary in the x-y plane of the cell. Providing an instrumental method for determining distance would not only allow analysis of the cell from the z axis, it would additionally provide a reproducible means for quantifying the distance at which measurements are taken, allowing comparison of multiple points on a single cell or among multiple cells. Our goal is the implementation of impedance feedback to measure and control the sample-probe distance. In the short-term this technology will be applied to the control of distance for flux measurements. In the long-term this technology will allow us to add scanning microscopy to our analytical toolbox.